A semiclassical model based on the Landauer transmission theory and McKelvey's flux theory is constructed to describe the field electron emission from folded graphene with an internal biased voltage. This model, which has taken the unique electronic energy band structure of monolayer graphene into account, leads to the explicit expression for drain to source and field emission line current density. The calculation results show that the field emission current density can be modulated by both the back gate(VB-gate) and Source-Drain(Vds) voltage under a certain anode electric field. According to the simulation, the field emission current density (J_FE) with Vds=0.8 V is twice as large as the one with Vds=0 when the external electric field at the tip is F=4 V/nm. The internal electric field tuned field emission provides a promising way of cathode modulation.